With increasing concerns of the greenhouse gas emission arising from electricity consumption and the rising energy cost, the demand for better thermal insulation in the building enclosures has drastically increased over the past few years. To improve the thermal insulation of building envelope, increasing the wall thickness is one of the solutions. However, it is less practical than directly reducing the overall thermal conductivity (k) of the envelopes of buildings.
For a given wall thickness, the thermal insulation of building envelope could be improved if the wall is constructed with foamed concrete with low thermal conductivity instead of normal concrete. Foamed concrete is a porous cementitious material formed by entrapping homogeneous pores into cementitious matrix using appropriate method. At present, introduction of pores can be achieved through mechanical means either by preformed foaming or mix foaming (Nambiar & Ramaurthy; 2007). The foaming agent for preformed foaming includes both the protein-based and synthetic-based foaming agent. Previous studies show that the thermal conductivity of concrete is usually proportional to its density (Shrivastava, 1977), and a decrease of dry density by 100 kg/m3 results in a reduction of thermal conductivity by 0.04 W/mK for lightweight aggregate foamed concrete (Weigler & Karl, 1980). Jones and McCarthy (2003) showed that foamed concrete with a plastic density of 1000 kg/m3 exhibited a typical thermal conductivity of 0.23-0.42 W/mK.
Since the strength of foamed concrete also decreases with increasing porosity, the strength of foamed concrete with sufficiently low thermal conductivity is always below the strength level for structural use. It would be desirable to develop a foamed concrete composition with both sufficiently low thermal conductivity and sufficient strength for structural use.
When foamed concrete is used to replace normal concrete, the presence of the pores will promote the penetration of moisture, chloride ion and carbon dioxide into foamed concrete and the durability against corrosion of steel reinforcement may be a concern. Previous studies showed that both the transport properties (including water permeability and chloride diffusivity) and carbonation resistance of foamed concrete are similar to those of normal concrete of similar strength (Chandra & Berntsson, 2003; Osborne, 1995). One important point to highlight, however, is that the test results are based on measurements on foamed concrete members that are not loaded and therefore not cracked. However, in practice, due to the low toughness of foamed concrete, it is easy for cracks to form in both foamed concrete and its protective coating/surface treatment (if applied) under loading. While the formation of fine cracks should not affect structural performance (as the tensile load capacity of concrete is neglected anyways), it can severely degrade the transport properties and carbonation resistance of foamed concrete (Chandra & Berntsson, 2003). Experimental findings in those studies actually indicated severe steel rusting at the vicinity of cracks in foamed concrete. With such a view, lightweight high performance fiber reinforced cementitious composites (FRCC) layers could be used, as a protective layer, together with foamed concrete. As there are no coarse aggregates used, the structure of FRCC can be designed as dense as that of normal concrete and even high strength concrete. More importantly, lightweight high performance FRCC can be designed to show high ductility, strain hardening and multiple cracking behaviors, and crack control capability under loading (Wang & Li, 2003). Indeed, previous studies showed that high performance FRCC has the ability to control crack openings to below 0.05 mm under loading (Li & Leung, 1992; Lepech & Li, 2009). According to Wang et al (1997) and Djerbi et al (2008), the water permeability and chloride diffusivity of concrete will not be affected by cracks that are so fine. In addition, with low density and thermal conductivity of lightweight FRCC, the thermal insulation performance of lightweight FRCC layer would be compatible to that of foamed concrete. It is hence possible to use lightweight FRCC layer to protect foamed concrete from external environmental factors under both loading and unloading conditions.
U.S. Pat. No. 6,969,423 discloses lightweight high performance fiber reinforced cementitious composite (FRCC) showing low density, high ductility and strain hardening as well as multiple cracking behaviors. However, both the thermal conductivity and transport properties of the lightweight FRCC are not disclosed.
With such a view, it would be desirable to develop a lightweight high performance FRCC layer with good thermal insulation and sufficient barrier resistance to moisture/chloride ion/carbon dioxide penetration, as a protective layer for foamed concrete.